Since you already have the UPS and EBM's then just roll with that. Since it is a 6kVA model, be sure you have it on a 30A 240V circuit with a dedicated breaker, even if you don't have a significant power draw day to day when things are fine, when the UPS recharges it will draw additional current to do the recharging, and if you have it on a normal 15A circuit it will trip the breaker when power comes back and starts to recharge the batteries, defeating the whole purpose
If you cable up your own battery strings, make sure the fuses are in each battery pack to prevent problems. The original wiring harness for those has the short spade connector jumpers to go between the adjacent batteries and a long one from the back of the pack to the anderson conenctor in the front + one of the jumpers has a built-in fuse. w/o that fuse, if a battery fails you can have it take out the entire system and at a higher risk of starting a fire.
The 6kVA UPS you have is a double-conversion model, so AC goes in, it goes to DC, charges batteries if needed, then the DC gets converted back to AC and output. If you look in your settings on the UPS, you can tune it to actually output 208 or 240V regardless of the input voltage. The high internal DC voltage is used to help minimize conversion losses. Since internally it is ~190V DC all the time it just pulls power from the batteries if the input AC goes away and you don't have the power interruption that you see with cheaper UPS units that will kick in the inverter and disconnect the load from the AC line when there is a power problem.
For inverter solutions like people mention above I would suggest starting looking at the solar / alternative energy sites because a big solar power install is very similar in topology to a large capacity UPS, with the added complexity of a maximum power point tracking module that pulls the power out of the solar panels in a way that gets the max effeciency out of the system, and work with the loads and available battery capacity to minimize losses. Find the company names on those sites and dig deeper and you will find their other products that are not as solar focused to get an idea of what is out there commercially.